1. Field of the Invention
The present invention relates to a method of and an apparatus for optically shaping a photo-setting resin such as an ultraviolet-curing resin into a desired resin model by applying a light beam to the photo-setting resin and scanning the same with the applied light beam, and more particularly to a method of and an apparatus for optically shaping, with high accuracy, a closed region of a photo-setting resin which is surrounded by a contour line.
2. Description of the Related Art
Attempts have heretofore been made to scan the surface of an ultraviolet-curing resin solution stored in a tank with an ultraviolet laser beam while turning on and off the ultraviolet laser beam thereby producing a scanned set layer of ultraviolet-curing resin, and successively stacking such scanned set layers into a three-dimensional desired resin model. Such a resin model is typically used as a master model for fabricating products. Therefore, it is necessary to increase the accuracy with which the ultraviolet-curing resin is shaped, the bonding strength between adjacent scanned set layers, and the efficiency with which the ultra-violet curing resin is shaped.
According to one optical shaping process, an ultra-violet laser beam is generated by an ultraviolet laser oscillator, turned on and off and scanned under the control of an optical system which has a galvanometer mirror and a shutter, and applied to the surface of an ultraviolet-curing resin solution stored in a tank. The tank accommodates therein a vertically movable elevator which can block the ultraviolet laser beam. A resin solution layer that is interposed between the surface of the ultraviolet-curing resin solution and the elevator is set by the ultraviolet laser beam applied thereto.
In the optical shaping process, the elevator is in a lifted position, and, a resin solution layer that is present between the surface of the ultraviolet-curing resin solution and the elevator is set by the ultraviolet laser beam applied thereto, thereby forming a first scanned set layer. Then, the elevator is lowered a distance corresponding to the first scanned set layer, and the scanning ultraviolet laser beam is applied to form a second scanned set layer over the first scanned set layer. Subsequently, scanned set layers are successively formed downwardly until a final scanned set layer is formed. After the final scanned set layer is formed, the elevator is lifted to remove a desired resin model, which is composed of the stack of scanned set layers thus produced, out of the resin solution. To finally set the resin model, the resin model, designated by W in FIG. 10 of the accompanying drawings, is irradiated with an ultraviolet radiation emitted from an ultraviolet lamp 50 for a long period of time.
A horizontal plane in the ultraviolet-curing resin solution within each vertical distance that the elevator traverses on its vertical movement is hereinafter referred to as an isometric section. One isometric section contains a region (a set region) where the resin solution is to be set and a region (an unset region) where the resin solution is not to be set, depending on the three-dimensional shape of the resin model W to be fabricated.
The ultraviolet laser beam generated by the ultra-violet laser oscillator is scanned along a scanning line over the resin solution layer in each isometric section by the optical system. The ultraviolet laser beam is turned on, i.e., a shutter (AOM) is open, within the set region, and is turned off, i.e., the shutter (AOM) is closed, within the unset region. When the scanning along one scanning line is over, the ultraviolet laser beam is shifted a scanning pitch, and then scanned along a next scanning line over the resin solution layer.
When the ultraviolet laser beam is applied to the ultraviolet-curing resin solution, the radiation energy thereof is progressively reduced as it enters the ultraviolet-curing resin solution. Therefore, from a microscopic viewpoint, the region of the ultraviolet-curing resin solution which is irradiated with the ultraviolet laser beam, i.e., the scanned set layer, contains sharp lower edges of set resin.
To form a stack of successive scanned set layers of ultraviolet-curing, resin, an upper scanned set layer is produced by an ultraviolet laser beam whose radiation energy is strong enough to irradiate a lower resin solution layer, so that the depth to which the resin solution is set across the upper-layer isometric section is greater than the actual thickness of the upper scanned set layer. In this manner, the successive scanned set layers of ultraviolet-curing resin are joined together with a large bonding strength.
If the scanned set layer in one isometric section is of a closed region enclosed by a contour line as shown in FIG. 11 of the accompanying drawings, then it is formed according to the conventional optical shaping process as follows: First, a contour line OL shown in FIG. 11 is recognized. Then, as shown in FIG. 12 of the accompanying drawings, the resin solution layer is scanned along a scanning line parallel to one edge OL1 of the contour from a start-of-scan point P0 to an end-of-scan point P1. Thereafter, the optical system is controlled to shift the scanning ultraviolet laser beam by a scanning pitch, and then the resin solution layer is scanned along a next scanning line from a start-of-scan point P2.
Such a scanning mode is referred to as a raster scanning mode. In the conventional optical shaping process which employs the raster scanning mode, the ultraviolet laser beam is applied as a circular beam spot to the ultraviolet-curing resin solution. Therefore, as shown in FIG. 12, the scanned set layer is not shaped exactly along the contour line at edges OL2, OL3 where the scanning lines start and end. Consequently, the conventional optical shaping process suffers problems in the production of highly accurate resin models.
To produce a three-dimensional resin model highly accurately according to the conventional optical shaping process, it is necessary to reduce the diameter of the ultraviolet laser beam spot and also to reduce the scanning pitch. However, reducing the spot diameter and the scanning pit results in an increase in the time required to produce the resin model or requires stricter beam position control at the start-of-scan and end-of-scan points.